lmao that's 100% an urban legend, another variation I heard from a Tufts student was because the foundation of widener library was so unstable that a subway train going too fast under mass ave would make the whole building shake. There's currently a much more severe slowdown than usual, but the normal slowdown is because the subway has to make a (relatively) sharp turn going between central and harvard. If you look at a map you'll see that the red line makes an almost 90 degree turn north to go towards porter square